Provisional U.S. Application for Patent 61/337,980 filed 14 Feb. 2010 of Xiaoming Chen et al. describes methods and apparatus for Global Navigation Satellite Systems (GNSS) signal processing with regional augmentation.
Trimble Navigation Limited has developed such a regionally augmented GNSS service based around a sparsely distributed ground network of GNSS reference stations covering the United States Midwest. GNSS observation data from the reference stations are concentrated at a network processing center where a regional correction signal is generated. The regional correction signal is then compressed and formatted for broadcast via L-band satellite link to users in the field.
The correction signals generated by the network server and distributed to users include the following components:                At least one code bias per satellite;        A phase-leveled geometric correction per satellite, derived from network fixed double-difference ambiguities;        An ionospheric delay per satellite for each of the multiple regional network stations and/or non-ionospheric corrections.        
The corrections enable construction of synthetic reference station (SRS) data for input into rover GNSS receivers, allowing the rover equipment to perform real-time kinematic (RTK) positioning. Generation of the synthetic reference station data can be performed at the rover or at a server given the approximate location of the rover.
The SRS correction stream mimics data from a physical base receiver and therefore can be processed in a manner that is very similar to standard single-base RTK position estimation using, for example, a factorized array of filters for multi-carrier ambiguity resolution (FAMCAR) as described in U.S. Pat. No. 7,432,853 or an equivalent big-filter approach. In a big-filter approach, all applicable states are modeled in one single filter, rather than in a factorized array of filters. This is discussed in more detail in a later section, entitled “SRS Data Processing using a Big Filter Approach”, below.
A discontinuity in tracking of the carrier-phase of GNSS signals at a receiver is referred to as a “cycle slip,” which can occur for a variety of reasons such as obstructions and weak signals. Techniques for detecting and in some cases repairing cycle slips are described for example in G. Seeber, Satellite Geodesy, 2d Ed., 2003 at pages 277-281.
A cycle slip in tracking a satellite at the rover and/or at a network reference station is directly related to a carrier, such as L1 or, more often, L2 in the case of GPS. However, as described in Provisional U.S. Application for Patent 61/337,980, the correction signals are based on combinations of network reference station observations.
Processing methods and apparatus capable of dealing with cycle slips in positioning with regional augmentation are desired, especially to achieve faster convergence to a solution, improved accuracy and/or greater availability.